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You were absolutely correct in your guess, these are people counters that use RadioBeam technology that we developed. There are ones similar to these on the paths up to Ben Nevis. This is quite an old unit and the modern ones are much smaller and can be supplied fitted into wooden or metal posts. You can find details on our web site.
The system works by sending out a beam of very high frequency radio waves from one side and detecting it on the other side of the path. As people walk in between the two units the radio signal is modified and circuitry detects this and increments a data logger. The advantage of this counter technology is that the beam can pass through plastic etc so that there are no external lenses etc to be kept clean and more importantly the system is completely unaffected by rain or snow.
We developed this counter for extreme conditions and they are used on hill paths and long distance routes in the UK. We also sell many units to Sweden for all year round use, they are used in ski resorts in the States and have been used in Alaska for counting moose at temperatures down to -40 C. Over the last few years the use of these counters has been extended to parks etc. as they can be made virtually vandal proof.
We also sell combined bicycle and people counters which use 2 radio frequency beams, one high frequency and one low, which differentiate between cyclists and walkers.
If you would like any further information please let me know.
Alasdair